LeCroy DDA 7 Zi series, Руководство оператора

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WavePro 7Zi
451
WP700Zi-OM-E-RevA
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Identify areas where two head signal differences are greatest and provide a benchmark for the difference
and the byte locations where these occur
The Channel Analysis feature can be operated in either of these basic modes:
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Channel Emulation
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Analog Compare
Because each requires a similar setup, both share the same Channel Setup menu system, which lets you switch
between modes without the need to redefine setup details.
Channel Emulation
Often when examining a partial response maximum likelihood (PRML) data signal, it may be desirable to view
those signal locations where the quality is poorest and the signal is likely to be the most problematic for a PRML
channel.
Channel Emulation enables analysis of PRML head signal quality from the point of view of the PRML channel.
This is made possible by the emulation of PRML channel processing by means of signal equalization, automatic
gain control (AGC), phase lock loop (PLL), sampling, and Viterbi detection.
For each PRML sample, a quality sequenced amplitude margin (SAM) benchmark is determined. The lower the
value of SAM the greater the difficulty for the drive channel to produce the right data value for that sample, and
the less margin there will be. A SAM value of less than zero indicates that an incorrect data value will be selected
at that sample.
PRML signals characteristically have a set of target-level values that the signal samples should meet at the
sampling times. For example, for a PR4 signal, the targets are
1 0 -1 . In order to achieve these desired levels, the
DDA’s channel emulation equalizes the head signal, either automatically or using selected values for -3 dB cutoff
and boost. The equalized head signal is then displayed. This lets you examine the head signal after it is
processed, when it will have the characteristic PRML waveshape.
The distance of the waveform samples from the PRML target values is a first-order indication of the quality of a
PRML signal. The channel emulation annotates the head signal after equalization with the target values. This
makes visual interpretation of the quality of the head signal possible and intuitive. As a result, you can visually
inspect the head signal to an extent beyond the more obvious indications of problems such as thermal asperities
and dropouts.
In addition, several powerful ways are available for selecting how the section of the equalized, annotated head
signal will be viewed:
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You can scroll through the head signal in the traditional time sequential mode using the Auto Scroll feature,
which provides "hands free" scrolling at a rate and in the direction that you specify.
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You can select which part of the head signal is to be displayed by byte number. This is particularly useful if
a data error is known to exist at a particular byte location.
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You can review the head signal in order of poorest SAM value. This capability is particularly practical
because the areas of poorest quality are generally of greatest interest.
The ways in which Channel Emulation can be operated are with or without a reference signal, and Stop on SAM
(on/off).
With or Without Reference
Using a reference head signal, where available, provides two major benefits. First, the head signal under analysis
can be viewed with a head signal that has a reference for improved interpretation of waveform misshapes. The
DDA will equalize both reference and head signals. It will also auto-align the two signals when selecting the head
signal to be viewed by SAM, even if they were captured at different spindle speeds (up to 1%).
An additional benefit is that the SAM calculation can then assume values less than zero and indicate a data error
likely at that point. Without a reference signal, it would be impossible to determine whether an error occurs for a
particular sample, only the confidence (SAM) of the Viterbi detector in selecting between a data ‘1’ or data ‘0’ for a
particular sample. In this case, the minimum confidence level possible is zero SAM, indicating no confidence in
making a selection. The correct data can be specified with a reference, which allows SAM values of less than
zero to be detected when the signal quality is so poor that an incorrect data value would be selected by the DDA's
Viterbi detector.